(a) Field of the Invention
The present invention relates to the method of producing boards for printed wiring which are used for the production of electrical and electronic equipment.
(b) Description of the Related Art
Boards for printed wiring to be used for the production of electrical and electronic equipment have been produced by heating and pressing by a press several prepregs comprising base materials, such as paper or glass cloth, impregnated with phenolic resins or epoxy resins, together with metal foil, which is to be a circuit conductor.
The properties of such boards are dominated substantially by the base materials which are present within the boards in a large ratio of 20 to 50% by volume, and it has been difficult to endow the boards with specific characteristics, such as a low thermal expansion, a high thermal conductivity or a high dielectric constant.
Another known method uses molding materials for the production of the boards, but when thermoplastic resins are used as the molding materials, there arise some problems, such as poor soldering thermal resistance, the use of costly thermal resistance resins and warp or deformation. Further, thermoplastic resins generally have high melt viscosities, which arrest the addition of a large amount of fillers for improving the properties of the boards remarkably. In Japanese Patent Application Kokai Koho (Laid-open) Nos. 53-50469 and 58-77276 are disclosed the use of thermosetting resins. However, in the method disclosed therein, conductor layers are provided by plating or bonding after the molding of boards, and it is somewhat doubtful whether the conductor layers have sufficient adhesion strength and electric properties. The integral molding of conductor foils and molding materials are disclosed, for example, in Japanese Patent Application Kokai Koho (Laid-open) No. 62-77926. However, there is no disclosure of the composition of the molding materials used, and inorganic fillers are used in working examples in such a small quantity of about 20% by volume that the boards may have a high coefficient of expansion, thereby deteriorating the adhesion and dimensional stability of the conductor layers (accuracy of circuits).
In Japanese Patent Application Kokai Koho (Laid-open) Nos. 55-57212, 61-136281 and 3-221448 are disclosed the methods for producing boards having high dielectric constant by incorporating high dielectrics into insulating layers. The insulating layers, however, also contain base materials, such as glass cloth, which disable incorporation of the high dielectrics in a large amount sufficient for varying the dielectric constant remarkably.
As described above, the conventional boards containing base materials, such as paper or glass cloth, are difficult to remarkably vary their properties, such as thermal expansion coefficient, thermal conductivity or dielectric constant. Molded boards containing no base materials also involve problems in thermal resistance, cost or reliability.